(a) Field of the Invention
This invention relates to tone level control in an electronic musical instrument, and more particularly, it pertains to touch-responsive control of the tone signal level in an electronic musical instrument.
(B) Description of the Prior Art
In an electronic musical instrument provided with a keyboard, the tone pitch is determined by the selection of the keys on the keyboard. Here, the tone level desirably is controlled by the speed of the depression of the desired key. Namely, it is desirable that, when a key is depressed strongly or at a high depression speed, a strong tone or a high level signal is generated, whereas when a key is depressed softly, a weak tone or a low level signal is generated. The correspondence between the key depression and the tone generation should desirably be held in the whole dynamic range.
In the conventional touch-responsive circuit desired for use in an electronic musical instrument, however, there exists a lower limit in the key depression for generating a tone. That is, in an extreme pianissimo performance, it may happen that no sound is generated from the instrument.
FIG. 1 shows an example of the conventional touch-responsive circuit designed for use in an electronic musical instrument provided with a keyboard. The circuit of FIG. 1 corresponds to one key and similar touch-responsive circuits are provided for the respective keys in the keyboard. In this figure, a tone generator TG generates a tone signal of a predetermined tone pitch corresponding to the key and a gate circuit GC (usually called keyer) controls, by a control signal, the signal level or the envelope of the tone signal to be delivered therefrom. The circuit for generating this control signal, i.e. the touch-responsive circuit, basically consists of a pair of charging and discharging circuits wherein the charging voltage for the pair's second circuit is controlled by the discharging voltage of the pair's first circuit and represents the speed of the key depression, i.e. the touch, and the voltage across a capacitor of the second circuit is fed to the gate circuit as the control signal.
A change-over key switch SW.sub.1 is interlocked with the key of the keyboard and has: one stationary (normally closed) contact connected to a dc voltage source E.sub.1, another stationary (normally open) contact connected to the base of a transistor Q.sub.1, and a movable contact A connected to a parallel connection of a capacitor C.sub.1 and a resistor R.sub.1. The dc voltage source E.sub.1, the change-over switch SW.sub.1 and the parallel connection of the capacitor C.sub.1 and the resistor R.sub.1 connected to the movable contact of the switch SW.sub.1 jointly constitute a key depression speed detector. More specifically, when the key is in its released state, the movable contact A is connected to the dc voltage source E.sub.1 as shown in FIG. 1 and the capacitor C.sub.1 is charged up to the source voltage E.sub.1. When the key is depressed, the movable contact A is turned over to the base of the transistor Q.sub.1. It should be understood that, during this part of operation, since the resistor R.sub.1 forms a discharging circuit for the capacitor C.sub.1, the voltage V.sub.cl across the capacitor C.sub.1 decreases with time and hence the initial voltage applied to the base of the transistor Q.sub.1 depends on how quickly the connection of the movable contact interlock with the key is changed over to the base of the transistor Q.sub.1, i.e. the initial voltage depends on the key depression (movable contact travelling) speed. A base resistor R.sub.2 is connected between the base of the transistor Q.sub.1 and the ground. The collector-emitter circuit of the transistor Q.sub.1 forms an electronic switch of a charging circuit for a capacitor C.sub.2 from a voltage source terminal V.sub.cc through a resistor R.sub.4. The resistor R.sub.4 serves to determine the charging time constant for the capacitor C.sub.2. As the resistor R.sub.4 is of a very small resistance value, the capacitor C.sub.2 is charged up rapidly. More particularly, when the movable contact A of the change-over switch SW.sub.1 is connected to the base of the transistor Q.sub.1, the voltage V.sub.cl across the capacitor C.sub.1 is applied to the base of the transistor Q.sub.1 and the transistor Q.sub.1 is turned "on" to an extent corresponding to the base potential. Then, the capacitor C.sub.2 is charged up to a voltage determined by the base potential of the transistor Q.sub.1 which represents the key depression speed. The voltage V.sub.cl across the capacitor C.sub.1 decreases due to the discharge through the resistors R.sub.1 and R.sub.2 (where R.sub.2 &gt;&gt; R.sub.1), and consequently the base potential of the transistor Q.sub.1 falls to the ground level so that the transistor Q.sub.1 is turned off. Thus the transistor Q.sub.1 and the capacitor C.sub.2 constitute a peak hold circuit of the voltage appearing at the movable contact A. Then, the charge stored in the capacitor C.sub.2 begins to discharge through the resistors R.sub.3 and R.sub.4. Resistors R.sub.3 and R.sub.4 determine the discharging time constant for the capacitor C.sub.2. Such a decaying voltage V.sub.c2 is applied to the gate circuit GC for determining the envelope of the tone signal. Thus, the gate circuit GC allows the passage of a tone source signal having a level or an envelope corresponding to the key depression speed, i.e. the touch.
In such a touch-responsive circuit, however, it should be understood that, when a key is depressed very slowly, the charge stored in the capacitor C.sub.1 may be discharged out substantially during the period that the movable contact A travels from the dc voltage source E.sub.1 to the base of the transistor Q.sub.1. Namely, when the key is depressed and the movable contact A is disconnected from the dc voltage source E.sub.1, the voltage V.sub.cl across the capacitor C.sub.1 begins to decrease as shown by the broken curve in FIG. 3. In FIG. 3, the ordinate represents the voltage V.sub.cl across the capacitor C.sub.1 (potential at the movable contact) and the abscissa represents "time" from the disconnection of the movable contact A off the dc voltage source E.sub.1 (i.e. key depression). Here, since the resistance of the base circuit of the transistor Q.sub.1 may be substantially large as compared with the resistance R.sub.1, the connection of the movable contact A to the base of the transistor may not alter the curve substantially. When the potential at the movable contact decreases below some level before the movable contact somes into contact with the base of the transistor Q.sub.1, the transistor Q.sub.1 does not become sufficiently conductive to charge the capacitor C.sub.2 to a level sufficiently high to open the gate circuit GC. Then, no tone signal is derived from the tone generator TG through the gate circuit GC even when the key is depressed.
More particularly, in the circuit of FIG. 1, let us now assume that the threshold base-to-emitter voltage of the transistor is V.sub.th (&gt;0), the voltage V.sub.cl at the movable contact A of the switch SW.sub.1 should not decrease below the threshold voltage V.sub.th to insure tone production. Since, however, the voltage V.sub.cl is arranged to drop from E.sub.1 toward zero through the resistor R.sub.1, treating the threshold voltage V.sub.th only as an arbitrary transient (passing point) voltage, a pianissimo performance may not generate a controlling voltage above the threshold voltage, and hence any sound will not be produced.